A major breakthrough in FCC catalysts and method of use came in the early 1960's with the introduction of molecular sieves or zeolites. These materials were incorporated into the matrix of amorphous and/or amorphous/kaolin materials constituting the FCC catalysts of that time. These new zeolitic catalysts, containing a crystalline aluminosilicate zeolite in an amorphous or amorphous/kaolin matrix of silica, alumina, silica-alumina, kaolin, clay or the like, were at least 1,000-10,000 times more active for cracking hydrocarbons than the earlier amorphous or amorphous/kaolin containing silica-alumina catalysts. This introduction of zeolitic cracking catalysts revolutionized the fluid catalytic cracking process. New innovations were developed to handle these high activities, such as riser cracking, shortened contact times, new regeneration processes, new improved zeolitic catalyst developments, and the like.
The new catalyst developments revolved around the development of various zeolites such as synthetic types X and Y and naturally occurring faujasites; increased thermal-steam (hydrothermal) stability of zeolites through the inclusion of rare earth ions or ammonium ions via ion-exchange techniques; and the development of more attrition resistant matrices for supporting the zeolites.
These zeolitic catalyst developments gave the petroleum industry the capability of greatly increasing throughput of feedstock with increased conversion and selectivity while employing the same units without expansion and without requiring new unit construction.
After the introduction of zeolite containing catalysts, the petroleum industry began to suffer from a lack of crude availability as to quantity and quality accompanied by increasing demand for gasoline with increasing octane values. The world crude supply picture changed dramatically in the late 1960's and 1970's. From a surplus of light, sweet crudes, the supply situation changed to a tighter supply with an ever increasing amount of heavier crudes with higher sulfur contents. These heavier and higher sulfur crudes presented processing problems to the petroleum refiner in that these heavier crudes invariably also contained much higher metals and Conradson carbon values, with accompanying significantly increased asphaltic content.
The effects of heavy metal and Conradson carbon on a zeolite containing FCC catalyst have been described in the literature as to their highly unfavorable effect in lowering catalyst activity and selectivity for gasoline production and their equally harmful effect on catalyst life.
Metal content and Conradson carbon are two very effective restraints on the operation of a FCC unit and impose restraints on a Reduced Crude Conversion (RCC) unit from the standpoint of obtaining maximum conversion, selectivity and catalyst life. Relatively high levels of these contaminants are highly detrimental to a catalytic conversion process. As metals and Conradson carbon levels are increased still further by available crude oils, the operating capacity and efficiency of a RCC unit and especially a FCC unit are adversely affected or even made uneconomical. These adverse effects occur even though there is enough hydrogen in the feed to produce an ideal gasoline consisting of a mixture of only toluene and isomeric pentenes (assuming a catalyst with such ideal selectivity could be devised).
The effect of increased Conradson carbon is to increase that portion of the feedstock converted to coke deposited on the catalyst. In typical gas oil cracking operations employing a crystalline zeolite containing catalyst in an FCC unit, the amount of coke deposited on the catalyst averages about 4-5 wt% of the feed. This coke production has been attributed to four different coking mechanisms, namely, contaminant coke from adverse reactions caused by metal deposits, catalytic coke caused by acid site cracking, entrained hydrocarbons resulting from pore structure adsorption and/or poor stripping, and Conradson carbon resulting from pyrolytic distillation of hydrocarbons in the conversion zone. There has also been postulated two other sources of coke present in reduced crudes in addition to the four above identified. They are: (1) adsorbed and absorbed high boiling hydrocarbons which do not vaporize and cannot be removed by normally efficient stripping, and (2) high molecular weight nitrogen containing hydrocarbon compounds adsorbed on the catalyst's acid sites. Both of these two types of coke producing phenomena add greatly to the complexity of resid oil processing. Therefore, in the processing of the higher boiling fractions or portions of crude oil, e.g., reduced crudes, residual fractions, topped crude, and the like, the coke production based on feed is the summation of the four types present in gas oil processing, plus coke from the higher boiling unvaporizable hydrocarbons and coke associated with the high boiling nitrogen containing molecules which are adsorbed on the catalyst. Coke production on clean catalyst, when processing reduced crudes, may be estimated as approximately 4 wt% of the feed plus the Conradson carbon value of the heavy feedstock, plus an additional correction factor related to % of feed boiling above 1050.degree. F. and % nitrogen in the feed.
The coked catalyst is brought back to equilibrium activity by burning off the deactivating coke in a regeneration zone in the presence of air, and the regenerated catalyst is recycled back to the reaction zone. The heat generated during regeneration is removed in part by the catalyst and carried to the reaction zone for vaporization of the feed and to provide heat for the endothermic cracking reaction. The temperature in the regenerator is normally limited because of metallurgical limitations and the hydrothermal stability of the catalyst.
The hydrothermal stability of a crystalline zeolite containing catalyst is determined by the temperature and steam partial pressure at which the crystalline zeolite begins to rapidly lose its crystalline structure and to yield a lower activity amorphous material. The presence of steam in high temperature operating modes is highly critical and is generated by the burning of adsorbed and absorbed (sorbed) carbonaceous material which has a significant hydrogen content (hydrogen to carbon atomic ratios generally greater than about 0.5). This carbonaceous material is principally the high boiling sorbed hydrocarbons with boiling points as high as 1500.degree.-1700.degree. F. or above that have a modest hydrogen content and the high boiling high molecular weight nitrogen containing hydrocarbons, as well as related porphyrins and asphaltenes. The high molecular weight nitrogen compounds usually boil above 1025.degree. F. and may be either basic, acidic or neutral in nature. The basic nitrogen compounds may neutralize acid sites while those that are more acidic may be attracted to metal sites on the catalyst. The porphyrins and asphaltenes also generally boil above 1025.degree. F. and may contain elements other than carbon and hydrogen. As used in this specification, the term "heavy hydrocarbons" includes all carbon and hydrogen containing compounds that do not boil below about 1025.degree. F., regardless of whether other elements are also present in the compound.
The heavy metals in the feed are generally present as porphyrins and/or asphaltenes. However, certain of these metals, particularly iron and copper, may be present as the free metal or as inorganic compounds resulting from either corrosion of process equipment or contaminants from other refining processes.
As the Conradson carbon value of the feedstock increases, coke production increases and this increased load will raise the regeneration temperature; thus the unit may be limited as to the amount of feed that can be processed because of its Conradson carbon content. A new development in reduced crude processing as described in pending U.S. applications referenced below, can operate at regenerator temperatures in the range of 1350.degree. up to 1600.degree. F. But even these higher regenerator temperatures place a limit on the Conradson carbon value of the feed at approximately 8, which yields about 12-13 wt% coke on the catalyst based on the weight of the feed. This level is controlling unless considerable water is introduced to further control temperature.
The metal containing fractions of reduced crudes contain Ni-V-Fe-Cu in the form of porphyrins and asphaltenes. These metal containing hydrocarbons are deposited on the catalyst during processing and are cracked to some extent to deposit the metal on the catalyst or are carried over by the coked catalyst as the metallo-porphyrin or asphaltene and converted by burning to the metal oxide during regeneration. The adverse effects of these metals as taught in the literature are to cause non-selective or degradative cracking and dehydrogenation to produce increased amounts of coke and light gases such as hydrogen, methane and ethane. These mechanisms adversely affect selectivity, resulting in poor yields and quality of gasoline and light cycle oil. The increased production of light gases, while impairing the yield and selectivity also have an undesirable effect on the gas compressor capacity. The increase in coke production, in addition to its negative impact on yield, also adversely affects catalyst activity-selectivity, greatly increases regenerator air demand and compressor capacity, and may result in uncontrollable and/or dangerous regenerator temperatures.
These problems of the prior art have been greatly minimized by the development of a new process which can handle reduced crudes or crude oils containing high metals and Conradson carbon values previously not acceptable for direct FCC processing. Normally, the less desirable crudes require expensive vacuum distillation and other treatments to isolate suitable metals free feedstocks and produce as a by-product, sulfur containing vacuum still bottoms. However, certain crude oils such as Mexican Mayan or Venezuelan crude oils contain abnormally high metals and Conradson carbon values. If these poor grades of crude are processed directly in a catalytic cracking process, they will lead to an uneconomical operation because of the high burning load imposed on the regenerator to remove carbonaceous deposits catalyst deactivation by metals and a high catalyst addition rate required to maintain catalyst activity and selectivity. The addition rate can be as high as 4-8 lbs./bbl. or more which at today's catalyst prices, can add as much as $2-8/bbl. of additional catalyst cost to the processing economics. It is thus desirable to develop and identify an economical means of processing more of the poor grade crude oils, such as a Mexican Mayan, because of their availability and relative cost as compared to Middle East crudes.
The literature suggests many processes for the reduction of the metals content and Conradson carbon values of reduced crudes and other contaminated oil fractions. One such process is that described in U.S. Pat. No. 4,243,514 and German Pat. No. 29 04 230 assigned to Englehard Minerals and Chemicals, Inc., which patents are incorporated herein by reference. Basically, these prior art processes involve contacting a reduced crude fraction or other contaminated oil with sorbent material at elevated temperatures in a sorbing zone, such as a fluid bed, to produce a product of reduced metal and reduced Conradson carbon value. One of the sorbents described in U.S. Pat. No. 4,243,514 is an inert solid initially composed of kaolin, which has been spray dried to yield microspherical particles having a surface area below 100 m.sup.2 /g and a catalytic cracking micro-activity (MAT) value of less than 20 which material is subsequently calcined at high temperature so as to achieve better attrition resistance. As the vanadia content on such sorbents increases above 5000 ppm and into the range of 10,000-30,000 ppm, the sorbent begins to have fluidization problems and more importantly, coking plugging of the reactor riser which have been overcome previously by removal of most of the spent sorbent inventory and addition of fresh virgin material in place thereof. This plugging may require shutting down the sorbent contacting facility.